The stepfamily is becoming an increasingly common family constellation in America. While many such families adapt well, others are at risk for both marital and child adjustment problems. Remarried couple are 25% more likely than first marrieds to divorce (Martin & Bumpass, 1989) with 50% of the divorces occurring in the first five years. Factors associated with each family structure transition place the children at risk for problems (Capaldi & Patterson, 1991). Stepfamilies need effective strategies to overcome the odds against them. It is shocking but true that there are few carefully evaluated intervention programs specifically designed to strengthen stepfamilies and none that address managing child behavior problems in stepfamilies (Lawton & Sanders, 1994). A randomized experimental longitudinal design is proposed with newly married stepfamilies who have children with early signs of conduct problems indicative of risk for later conduct disorder. The study has two foci: preventive intervention development and research and basic theory development. For the intervention aspect of the study, group design will be employed to provide an estimate of the utility and efficacy of the intervention across participants. Single-case methodology will furnish a rich data set for the investigation of differential processes for boys and girls in stepfamilies, will facilitate the study of intervention process, and will test hypotheses about the sequence and pattern of change. Both components of the study will test models about the etiology of marital distress and children's conduct problems through manipulation of the hypothesized controlling variables. Multiple method assessment at baseline, termination, and 9- and 18-month follow up will be employed to ensure robust tests of the intervention and theoretical models. The sample will be composed of 132 recently married stepfamilies. Primary target children will be boys in grade Kindergarten through 3 who are the biological children of the participant mother, and who reside with the mother at least 50% of the time. Girls in 12 families in the single-case component of the study also will participate. The proposed intervention builds upon two effective social-learning-based programs, one that strengthens parenting practices to reduce children's conduct problems (e.g., Patterson, Dishion, & Chamberlain, 1993; Reid & Patterson, 1991), and one that strengthens marital practices to improve marital adjustment (Christensen, Jacobsen, & Babcock, in press; Jacobson & Addis, 1993). These programs will be combined and tailored for the specific needs of remarried couples with children exhibiting behavior problems. The goal of the intervention is to ameliorate distress at incipient levels and prevent its development into adjustment problems.